Australian C4 grass cover percentage

Brief description

These data describe the percent of the total grass foliage cover that is comprised of the cover from C4 grasses. It is a long-term average (2001-2021) derived from MODIS imagery. Percent values range between 10 to 95%. Data have a spatial resolution of 0.0025 decimal degrees (~250 m). No quantitative validation has been performed but only a qualitative comparison to site and mapped information. Note that this is not the foliage cover of C4 grasses, nor the relative abundance of C4 plants, but the percent of total grass foliage cover that comes from C4 plants. So, for example, if total grass foliage cover is 60% and the C4 percentage is 80%, then the C4 foliage cover would be 48% (i.e., 60% x 0.8) and the C3 grass foliage cover would be 12%.
Lineage: The C4 percent data have been derived from MOD09Q1 (collection 6) NDVI data. 16-day NDVI data were converted to percent foliage cover using the method of Donohue et al. (2014) and then split into separate tree and grass (persistent and recurrent) components following Donohue et al. (2009). The long-term average grass cover for each 16-day period was calculated (2001 to 2020) and were then used to identify the average day-of-year (DOY) of maximum cover. This average timing was used to estimate the percent of C4 cover based on the understanding that C3 grasses have peak growth (cover) in late autumn/early summer, and C4 in late summer. Using Tasmanian native grasslands (10% C4) and Queensland’s Mitchel grasslands (95% C4) as end points (Hattersley, 1983), the timing of minimum and maximum C4 cover was determined as DOY 17 and 307, respectively. The percent of C4 was then scaled linearly between these two end points.

Locations with peak growth over the austral summer between these two dates were assigned to either minimum or maximum C4 percent depending on whether the average January air temperature was below or above 22C, respectively, according to Collatz et al. (1998). The summer cropping regions of NSW and Queensland are the main areas where locations fell between these two dates and represent the regions of lowest accuracy. It is also known that areas that experienced a land cover change from or to plantation within the data period are unreliable. An additional condition was introduced to force alpine grasslands to have minimal C4 percent values. This was done by changing the date of peak C4 cover from DOY 17 to DOY 65 wherever January air temperatures were below 19C.

Collatz, G.J., Berry, J.A., & Clark, J.S. (1998). Effects of climate and atmospheric CO2 partial pressure on the global distribution of C4 grasses: present, past, and future. Oecologia, 114, 441-454.
Donohue, R.J., McVicar, T.R., & Roderick, M.L. (2009). Climate-related trends in Australian vegetation cover as inferred from satellite observations, 1981–2006. Global Change Biology, 15, 1025-1039. DOI: 10.1111/j.1365-2486.2008.01746.x
Donohue, R.J., Hume, I.H., Roderick, M.L., McVicar, T.R., Beringer, J., Hutley, L.B., Gallant, J.C., Austin, J.M., van Gorsel, E., Cleverly, J.R., Meyer, W.S., & Arndt, S.K. (2014). Evaluation of the remote-sensing-based DIFFUSE model for estimating photosynthesis of vegetation. Remote Sensing of Environment, 155, 349-365. DOI: 10.1016/j.rse.2014.09.007.
Hattersley, P.W. (1983). The distribution of C3 and C4 grasses in Australia in relation to climate. Oecologia, 57, 113-128.